[ntp:questions] Single GPS/PPS time source gets marked as a falseticker
unruh at wormhole.physics.ubc.ca
Tue Jul 12 21:24:26 UTC 2011
On 2011-07-12, Michael Eder <meder at whoi.edu> wrote:
> We are using a Garmin GPS 16x. I should clarify these are platforms in the
18x I presume.
> middle of the ocean so the sole time source is the GPS and PPS.
Are they really waterproof enough to leave them out in the salty ocean?
> We have measured the time delta between the GPS and the PPS and the GPS
> while delayed to process the NMEA string has a relatively low jitter and is
> quite reliable.
What is "relatively low" for you? 10 ms? 1usec?
> -----Original Message-----
> From: questions-bounces+meder=whoi.edu at lists.ntp.org
> [mailto:questions-bounces+meder=whoi.edu at lists.ntp.org] On Behalf Of Rob
> Sent: Tuesday, July 12, 2011 1:25 PM
> To: questions at lists.ntp.org
> Subject: Re: Single GPS/PPS time source gets marked as a
> Michael Eder <meder at whoi.edu> wrote:
>> Hi Folks,
>> For our buoy deployments we have GPS and a highly accurate oscillator
>> conditioned by the GPS PPS. Unfortunately when we let this run for
>> longer periods we see both the GPS and PPS being marked as false
>> tickers. There is a lot of information on the web about this, but I
>> have not found a definitive solution. What is the best way to configure
> NTP when you only
>> have a GPS NMEA string and pulse per second? In our situation it is
>> feasible that the NMEA may not be always be available while the PPS
>> is, we are seeing a problem when both the NMEA and pulse seem to be
>> working properly.
> Are you using gpsd?
> If so, please use the binary protocol for the GPS receiver in use instead of
> NMEA. NMEA often has problems with the timestamps.
> (in fact, NMEA says the timestamps are time-of-fix which does not have to be
> the current time but can be some time in the near past)
> I see you already have been tweaking the NMEA time to compensate for
> problems. This does not work well with gpsd. When you have a systematic
> time offset for NMEA you should try to solve it either by stopping the use
> of NMEA or by fixing the problem in the receiver with a firmware update.
> E.g. there is a recent Garmin receiver that has unusable NMEA time.
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